Our research program has two main components. Firstly, to develop and test models of cognitive processes via study of the way these processes breakdown following brain injury or disease. Secondly, to evaluate the cognitive status of psychiatric patients in order to test hypotheses concerning possible neuroanatomic correlates of these disorders. Our studies of semantic memory in patients with Alzheimer's disease suggest that posterior cortical pathology results in a selective degradation of previously acquired knowledge. These degraded knowledge representations are, in turn, proposed to be responsible for work-finding problems and to substantially contribute to poor memory in patients with Alzheimer's disease. Functional brain imaging studies of normal individuals using positron emission tomography (PET) have begun to elucidate the neural networks that mediate semantic knowledge in the normal human brain. These studies suggest that our knowledge of objects is stored as discrete cortical networks in which different types of attributes are stored near the regions of the brain that mediate the perception of these attributes. Studies of normal elderly individuals have concentrated on the effects of aging on implicit and explicit measures of memory for over-learned and novel information. These studies suggest that aging does not affect the ability to monitor how often events occur when the acquisition of this information is tested with implicit measures of memory. Our studies of relatively early-stage, medically asymptomatic individuals infected with the Human Immunodeficiency Virus (HIV) have continued to document subtle cognitive dysfunction and the relationship between this dysfunction and biological parameters, including EEG, MRI, and concentrations of a potent neurotoxin, quinolinic acid. Our studies of patients with obsessive-compulsive disorder (OCD) suggest that, although these subjects can acquire and store information normally, they may be overly attentive to certain aspects of the environment, which, in turn, may interfere with information processing.